The present invention relates to a method for fabricating a semiconductor device having a device isolation insulating film, and more specifically to a method for fabricating a semiconductor device having a device isolation insulating film having an elevated device reliability, in particular, an elevated device isolation characteristics, by covering each device formation area with an oxidation-resistant film such as a nitride film just until a gate oxidation when different gate oxide films are formed on the same chip.
In general, in a MOS semiconductor device having insulated-gate field effect transistors (MOS transistors) formed on a semiconductor device chip, when a high-breakdown-voltage device-component having a high reliable breakdown voltage and a low-breakdown-voltage device-component needing a high speed operation for a high speed information processing are incorporated in a single semiconductor device chip, the high-breakdown-voltage device-component is required to have a thick gate oxide film and a thick field oxide film. On the other hand, the low-breakdown-voltage device-component is required to have a thin gate oxide film and a thin field oxide film which meet with a microminiaturization and a high speed operation of the semiconductor device.
In the prior art, as a method for forming field oxide films having different film thicknesses and gate oxide films having different film thicknesses on the same substrate in order to fabricate this type of semiconductor device, there was proposed to form a plurality of different gate oxide films by separately forming required oxide films by repetition of a gate oxidation and a wet etching (See Japanese Patent Application Pre-examination Publication No. JP-A-09-036243). Referring to FIGS. 1A to 1E, there are shown diagrammatic sectional views illustrating a process of this prior art method for forming two kinds of gate oxide film.
FIG. 1A illustrates a condition in which a device isolation oxide film 121 for electrically isolating between device-components is formed on a principal surface of a silicon substrate 300 and a thin oxide film 221 is formed on the surface of the silicon substrate 300 to protect the substrate in an ion implantation. Therefore, the principal surface of the silicon substrate 300 is divided into a plurality of active regions confined by the device isolation oxide film 121. As shown in FIG. 1A, the silicon substrate 300 has a first device formation area 521 needing a first gate oxide film having a large film thickness, as a high breakdown voltage circuit exemplified by a write circuit in an electrically writable read-only-memory (erasable PROM), and a second device formation area 522 needing a second gate oxide film having a small film thickness for a low breakdown voltage since a high breakdown voltage is not needed. The device isolation oxide film 121 can be formed by a LOCOS (local oxidation of silicon) process, or alternatively by a trench isolation forming a trench in a region in which the device isolation oxide film 121 is to be formed, and filling up the trench with an insulating material such as a silicon oxide.
Then, as shown in FIG. 1B, the thin oxide film 221 is removed from the surface of the silicon substrate 300 by a first wet etching. Here, the device isolation oxide film 121 is thinned or diminished because of this first wet etching, and becomes a device isolation oxide film 122 which is thinner than the device isolation oxide film 121.
Thereafter, as shown in FIG. 1C, a first gate oxide 222 is formed on the surface of the silicon substrate 300 by oxidation, so that an oxide film is formed on an exposed surface of the silicon substrate 300 in each active region surrounded by the device isolation oxide film 122.
Next, gate oxide films required in the device formation areas 521 and 522, respectively, are formed separately from each other. First, as shown in FIG. 1D, a resist 421 is deposited and patterned to expose the second device formation area 522 in which a thin gate oxide film is to be formed in the active region, and the first gate oxide film 222 is removed from the surface of the substrate 300 in the second device formation area 522 by a second wet etching using the resist 421 as a mask. In this second wet etching, the device isolation oxide film 122 is further thinned or diminished to become a device isolation oxide film 123 which is thinner than the device isolation oxide film 122.
Thereafter, as shown in FIG. 1E, the resist 421 is removed, and a second gate oxide film is formed by oxidation. In this process, the second gate oxide film formed on the silicon substrate exposed in the active region within the second device formation area 522 constitutes a thin gate oxide film 224, and an oxide film formed by an additional oxidation carried out on the first gate oxide film 222 remaining on the active region within the first device formation area 521, constitutes a thick gate oxide film 223.
With the above mentioned process, the thick gate oxide film 223 and the thin gate oxide film 224 which are required in the device formation areas 521 and 522, respectively, are formed in the device formation areas 521 and 522, respectively.
In the above mentioned process for forming the gate oxide films having different film thicknesses by repeating the etching and the gate oxidation in a so called multi-oxide process for the purpose of fabricating a single semiconductor device having different gate oxide film thicknesses on a semiconductor device chip, the device isolation oxide film 121 in the second device formation area 522 is subjected to two wet etchings, one of which is carried out for removing the thin oxide film 221 as shown in FIG. 1B, and the other of which is carried out for removing the first gate oxide film 222, with the result that the device isolation oxide film 121 is thinned to the device isolation oxide film 122 and then further thinned to the device isolation oxide film 123.
The above mentioned example has two different kinds of gate oxide film, but it could be easily understood to persons skilled in the art that, if the number of the kinds of gate oxide film is increased to three, or four, or more, the number of etchings correspondingly increases, and in a device formation area in which a gate oxide film is formed after a last wet etching, the device isolation oxide film is exposed to a corresponding number of wet etchings.
However, because of a frequent repetition of the oxidation and the wet etching, the following problems have been encountered.
A first problem is that a device isolation leak occurs because implanted ions (particularly, boron) penetrates through a thinned device isolation oxide film and because an inversion layer is created by an interconnection passing directly above the device isolation oxide film. In order to prevent this device isolation leak, it can be considered to increase an initial oxidation amount for the device isolation oxide film in a recess LOCOS, or alternatively to realize the device isolation oxide film by a shallow trench isolation. However, the former is a hindrance to microminiaturization because a diffused layer is destroyed by a bird""s beak. The latter aggravates the problem of a step difference which will be described next.
A second problem is that because the thickness of the device isolation oxide film is reduced from an initial oxidized condition by repeated wet etchings, a non-negligible step difference occurs at a boundary between a diffused layer and the device isolation oxide film, with the result that when a gate polysilicon is etched, a polysilicon adversely remains, which causes a short-circuiting.
A third problem is that because of the repetition of the substrate surface oxidation and the wet etching, the impurity concentration in the substrate surface is disturbed, with the result that an electrical characteristics (particularly, a threshold value of a transistor) becomes unstable.
A fourth problem is that because of the repetition of the substrate surface oxidation and the wet etching, the roughness of the substrate surface is increased with the result that an electrical characteristics is deteriorated.
The above mentioned various disadvantages are attributable to formation of oxide films unnecessary for the device-components. If the formed oxide film is unnecessary, it is necessary to remove the oxide film before the gate oxidation, so that the substrate surface is exposed. At this time, it is necessary to remove the thickness corresponding to the oxide film to be removed plus an over-etching. In particular, in a thermal oxidation such as the gate oxidation, the thickness of the thick oxide film such as the device isolation oxide film is hardly increased by the oxidation of a degree sufficient to form the gate oxide film on the substrate. As a result, the device isolation oxide film is thinned by the thickness corresponding to the thickness removed by etching the oxide film formed on the substrate.
Accordingly, it is an object of the present invention to provide a method for fabricating a semiconductor device having a device isolation insulating film, which has overcome the above mentioned problems of the prior art.
Another object of the present invention is to provide a method for fabricating a semiconductor device having gate oxide films of different film thicknesses and a device isolation insulating film having an elevated device isolation characteristics.
The above and other objects of the present invention are achieved in accordance with the present invention by covering a substrate surface in each device formation area with a thin oxide film or an oxidation-resistant film such as a nitride film just until a respective gate oxidation in the process of forming different gate oxide films on a single semiconductor device chip, so that it is possible to prevent an unnecessary oxide film from being formed on the substrate, with the result that it is possible to reduce the amount of oxide film etchings carried out before the gate oxidation.
According to a first aspect of the present invention, there is provided a method fabricating a semiconductor device having a device isolation insulating film, the method comprising the steps of preparing a semiconductor substrate having a principal surface which is divided into a plurality of active regions confined by a device isolation insulating film and covered with an oxide film, the principal surface of the semiconductor substrate having at least first and second device formation areas each of which includes at least one active region, forming an oxidation-resistant film on the whole of the principal surface of the semiconductor substrate, selectively etch-removing the oxidation-resistant film and the oxide film using as a mask a first resist film formed on the semiconductor substrate to expose the first device formation area, forming a first gate oxide film on the semiconductor substrate, selectively etch-removing the oxidation-resistant film and the oxide film using as a mask a second resist film formed on the semiconductor substrate to expose the second device formation area, and forming a second gate oxide film on the semiconductor substrate. The oxidation-resistant film can be formed of a nitride film.
With the above mentioned arrangement, first, the oxidation-resistant film is formed on the oxide film formed on the principal surface of the semiconductor substrate in each active region, and the oxidation-resistant film thus formed is used as a protection film in each device formation area until in the same device formation area the oxidation-resistant film and the oxide film are removed and a gate oxide film is formed on the principal surface of the semiconductor substrate. Therefore, an unnecessary oxide film is in no way formed on the principal surface of the semiconductor substrate. In other words, when the first gate oxide film is formed on the principal surface of the semiconductor substrate in the first device formation area, since the principal surface of the semiconductor substrate in the second device formation area is covered with the oxidation-resistant film, an unnecessary first gate oxide film is not formed on the principal surface of the semiconductor substrate in the second device formation area. Therefore, a step of removing the first gate oxide film in the second device formation area, resulting in a simultaneous diminishment of the device isolation insulating film within the second device formation area, is no longer necessary. As a result, a sufficient thickness of the device isolation insulating film can be ensured. Accordingly, the device isolation characteristics is elevated, and reliability of the semiconductor device is correspondingly elevated.
In addition, in the first device formation area, a gate oxide film is formed by an additional oxidation for the second gate oxide film carried out on the first gate oxide film Therefore, the gate oxide film having a thickness smaller than that of the gate oxide film formed in the first device formation area is formed in the second device formation area. Thus, gate oxide films having different film thicknesses can be formed while preventing the thinning of the device isolation insulating film in the respective device formation areas.
According to a second aspect of the present invention, there is provided a method fabricating a semiconductor device having a device isolation insulating film, the method comprising the steps of preparing a semiconductor substrate having a principal surface which is divided into a plurality of active regions confined by a device isolation insulating film and covered with an oxide film, the principal surface of the semiconductor substrate having at least first and second device formation areas each of which includes at least one active region, selectively etch-removing the oxide film using as a mask a first resist film formed on the semiconductor substrate to expose the first device formation area, forming a first gate oxide film on the semiconductor substrate, selectively etch-removing the oxide film using as a mask a second resist film formed on the semiconductor substrate to expose the second device formation area, and forming a second gate oxide film on the semiconductor substrate. The device isolation insulating film can be formed by a LOCOS process, and can be embedded in a trench formed in the principal surface of the semiconductor device.
With the above arrangement, the oxide film is used as a protection film in each device formation area until in the same device formation area the oxide film is removed and a gate oxide film is formed on the principal surface of the semiconductor substrate. Therefore, when the first gate oxide film is formed on the principal surface of the semiconductor substrate in the first device formation area, since the principal surface of the semiconductor substrate in the second device formation area is covered with the already existing oxide film, the growth of the first gate oxide film in the second device formation area is suppressed by the already existing oxide film. Furthermore, when the second gate oxide film is formed, a gate oxide film is formed in the first device formation area by an additional oxidation for the second gate oxide film carried out on the first gate oxide film in the first device formation area. Therefore, the gate oxide film having a thickness smaller than that of the gate oxide film formed in the first device formation area is formed in the second device formation area. Thus, gate oxide films having different film thicknesses can be formed.
In addition, the etching amount required for removing the oxide film formed on the principal surface of the semiconductor substrate in the second device formation area becomes small because the growth of the first gate oxide film in the second device formation area is suppressed as mentioned above. As a result, it is possible to prevent the thinning of the device isolation insulating film. Accordingly, the device isolation characteristics is elevated, and reliability of the semiconductor device is correspondingly elevated.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.